Atomic delocalization as a microscopic origin of two-level defects in Josephson junctions

DuBois, Timothy C.; Russo, Salvy P.; Cole, Jared H.

doi:10.1088/1367-2630/17/2/023017

Cited by 13 publications

(26 citation statements)

References 43 publications

(95 reference statements)

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“…The structural and nanochemical properties of AlOx layers have a significant influence on the performance of these devices. For example, AlOx-layer thickness variations and structural defects in AlOx-tunnel barriers of JJs cause noise and limit the detection sensitivity of superconducting interference devices and coherence times in quantum bits [14][15][16][17][18][19][20].…”

Section: Introductionmentioning

confidence: 99%

Structural and nanochemical properties ofAlOxlayers inAl/AlOx/Al-layer systems

Fritz

Radtke

Schneider

et al. 2019

Phys. Rev. Materials

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The structural and nanochemical properties of thin AlOx layers are decisive for the performance of advanced electronic devices. For example, they are frequently used as tunnel barriers in Josephson junction-based superconducting devices. However, systematic studies of the influence of oxidation parameters on structural and nanochemical properties are rare up to now, as most studies focus on the electrical properties of AlOx layers. This study aims to close this gap by applying transmission electron microscopy in combination with electron energy loss spectroscopy to analyze the structural and nanochemical properties of differently fabricated AlOx layers and correlate them with fabrication parameters. With respect to the application of AlOx as tunnel barrier in superconducting Josephson junctions, Al/AlOx/Al-layer systems were deposited on Si substrates. We will show that the oxygen content and structure of amorphous AlOx layers is strongly dependent on the fabrication process and oxidation parameters.Dynamic and static oxidation of Al yields oxygen-deficient amorphous AlOx layers, where the oxygen content ranges from x = 0.5 to x = 1.3 depending on oxygen pressure and substrate temperature. Thicker layers of stoichiometric crystalline -Al2O3 layers were grown by electron-beam evaporation of Al2O3 and reactive sputter deposition.

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Section: Introductionmentioning

confidence: 99%

Structural and nanochemical properties ofAlOxlayers inAl/AlOx/Al-layer systems

Fritz

Radtke

Schneider

et al. 2019

Phys. Rev. Materials

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“…These suggestions include tunneling between inversion symmetric (e.g. examples A and B in figure 1 of [48]) and inversion asymmetric (e.g. example C in figure 1 of [48]) states.…”

Section: Experimental Relevancementioning

confidence: 99%

“…examples A and B in figure 1 of [48]) and inversion asymmetric (e.g. example C in figure 1 of [48]) states. Since defect-phonon coupling in amorphous solids is expected to be an order of magnitude larger than in disordered crystals [26,[50][51][52], our results suggest that the tunneling amplitude of inversion asymmetric TLSs, such as in example C in figure 1 of [48] for KCl (from [56]) and the geometry factor [23] is = f 1 5 t .…”

Section: Experimental Relevancementioning

confidence: 99%

“…example C in figure 1 of [48]) states. Since defect-phonon coupling in amorphous solids is expected to be an order of magnitude larger than in disordered crystals [26,[50][51][52], our results suggest that the tunneling amplitude of inversion asymmetric TLSs, such as in example C in figure 1 of [48] for KCl (from [56]) and the geometry factor [23] is = f 1 5 t . With these parameters the Debye-Waller exponent equation (A.1) can be determined keeping in mind that the longitudinal speed of sound is larger and the transversal one and thus contributions from longitudinal phonon branches are negligible in equation (A.1).…”

Section: Experimental Relevancementioning

confidence: 99%

“…Thus, the Polaron transformation T commutes with the biases and accordingly leaves them unmodified. the four-fold local degenerate state of an oxygen atom in a symmetrically pulled out aluminum cage [48], our results suggest that whereas bare tunneling lifts the four-fold degeneracy via dominant edge tunneling of the oxygen atom, in the presence of strong defect-phonon coupling the diagonal tunneling will dominate, and two two-fold degenerate states will result (two three-fold or four-fold degenerate states in three-dimensions, depending on the off-center direction of the localized states). Similar considerations can be applied also to quantum bits made of single crystal Al O 2 3 tunnel barriers [49] where tunneling TLSs may be attributed to oxygen atoms out of crystalline position at the interface layer.…”

Section: Experimental Relevancementioning

confidence: 99%

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Symmetry reduction for tunneling defects due to strong couplings to phonons

Nalbach

Schechter

2017

New J. Phys.

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Tunneling two-level systems (TLSs) are ubiquitous in amorphous solids, and form a major source of noise in systems such as nano-mechanical oscillators, single electron transistors, and superconducting qubits. Occurance of defect tunneling despite their coupling to phonons is viewed as a hallmark of weak defect-phonon coupling. This is since strong coupling to phonons results in significant phonon dressing and suppresses tunneling in two-level tunneling defects effectively. Here we determine the dynamics of a tunneling defect in a crystal strongly coupled to phonons incorporating the full 3D geometry in our description. We find that inversion symmetric tunneling is not dressed by phonons whereas other tunneling pathways are dressed by phonons and, thus, are suppressed by strong defectphonon coupling. We provide the linear acoustic and dielectric response functions for a tunneling defect in a crystal for strong defect-phonon coupling. This allows direct experimental determination of the defect-phonon coupling. The singling out of inversion-symmetric tunneling states in single tunneling defects is complementary to their dominance of the low energy excitations in strongly disordered solids as a result of inter-defect interactions for large defect concentrations. This suggests that inversion symmetric TLSs play a unique role in the low energy properties of disordered solids.

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Revealing Temperature-Dependent Noise Sources in Aluminum Oxide Josephson Junctions Using Topological Analysis

Han¹,

Sun²,

Hou

et al. 2022

J. Phys. Chem. C

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Despite many improvements in the quality and reliability of Josephson junctions, the understanding of their noise sources, particularly the effect of oxidation parameters on the atomic arrangement of the interface and barrier layers, remains elusive. Here, we apply a Voronoi tessellation, a geometrically structural and topological analysis, to the amorphous barriers in aluminum oxide junctions. To enable this analysis, we perform million-atom molecular dynamics simulations to develop oxidation models at different temperatures. We find that the temperature introduces noise by changing the atomic compactness of oxides in the junctions. High-temperature oxidation aggravates the structural disorder and surface roughness of the barrier. This work will pave the way for illustrating the microscopic noise origin of amorphous oxides, which can transform our fundamental understanding of noncrystalline materials and qubit decoherence mechanisms.

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Atomic delocalization as a microscopic origin of two-level defects in Josephson junctions

Cited by 13 publications

References 43 publications

Structural and nanochemical properties ofAlOxlayers inAl/AlOx/Al-layer systems

Structural and nanochemical properties ofAlOxlayers inAl/AlOx/Al-layer systems

Symmetry reduction for tunneling defects due to strong couplings to phonons

Revealing Temperature-Dependent Noise Sources in Aluminum Oxide Josephson Junctions Using Topological Analysis

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